Is my text uploaded anywhere?

No. The shift is applied entirely in your browser; nothing is sent to a server.

What's the difference between ROT13 and a Caesar cipher?

ROT13 is a Caesar cipher with the shift fixed at 13 — the point at which the alphabet's 26 letters split exactly in half, making it its own inverse. This tool supports any shift from 0 to 25, with ROT13 as the default.

Are spaces, numbers and punctuation affected?

No — only letters A–Z and a–z are shifted. Everything else, including spaces, digits and punctuation, passes through unchanged, and letter case is always preserved.

Is this cipher actually secure?

No — a Caesar shift has only 26 possible keys and is trivial to brute-force. It's used for light obfuscation (spoiler text, puzzle hints) and as a teaching example, never for real security.

ROT13 & Caesar Cipher

Apply ROT13 or a custom Caesar cipher shift to text, in either direction. Letter case and punctuation are preserved. Runs entirely in your browser.

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Worked examples

ROT13 — the classic shift-13 obfuscation
ROT13 is just a Caesar cipher with a fixed shift of 13 — applying it a second time with the same shift always returns the original text, since 13 + 13 wraps a full 26-letter alphabet.
A custom shift of 3, encoding a phrase
The original Caesar cipher used a shift of 3 — letters wrap from z back to a, and spaces and punctuation are always left unchanged.
Decoding a shift-3 message back to plain text
Decoding shifts letters the opposite direction by the same amount, recovering the exact original text for any shift value, not just 13.

What this tool does

This tool applies a Caesar-style shift cipher to text: every letter is replaced by the letter a fixed number of positions further along the alphabet, wrapping from z back to a when it runs past the end. ROT13 — a shift of exactly 13 — is included as the default, but any shift from 0 to 25 works, in either the encode or decode direction.

When you need it

Hiding spoiler text, puzzle answers or joke punchlines in a forum post or document, the classic use of ROT13 in online communities since long before it had a dedicated tool.
Decoding a ROT13'd message or classic Caesar cipher passage for a puzzle, CTF challenge or cryptography exercise.
Demonstrating how a substitution cipher works for a class or a technical interview, where seeing the letter-by-letter shift with a chosen amount makes the mechanism concrete.
Quickly obfuscating a piece of text so it isn't immediately readable at a glance — for example, in a support ticket or shared note where you don't want the literal content to jump out before someone deliberately looks at it.

Why ROT13 is its own inverse

ROT13 is a Caesar cipher with the shift fixed at 13, which is exactly half of the alphabet's 26 letters. Applying the same shift twice moves a letter forward by 26 positions total — a full lap of the alphabet — which lands it back where it started. That's why running ROT13-encoded text through ROT13 again always recovers the original: encoding and decoding are the identical operation for that one specific shift. No other useful shift value has this property (a shift of 0 changes nothing at all); for any other amount, decoding requires shifting the opposite direction by the same number of positions, which is why this tool has a separate direction toggle.

What gets shifted and what doesn't

Only letters A–Z and a–z are affected. Every other character — digits, punctuation, spaces, symbols — passes through completely unchanged, and letter case is always preserved (H becomes another uppercase letter, e another lowercase one). This is why ROT13'd text stays readable as text: word boundaries, sentence punctuation and capitalization all stay exactly where they were, only the letters themselves change.

This is obfuscation, not encryption

A Caesar shift has exactly 26 possible keys, one of which does nothing — trivial to brute-force by hand, let alone by computer, and just as trivial to break by simple letter-frequency analysis. It provides no real security and was never intended to; it dates back to Julius Caesar's use for battlefield messages, when interception itself was the hard part. Today it's used for light, voluntary obfuscation and as a first teaching example for substitution ciphers, never for anything that needs to resist a deliberate attempt to read it.

Limits

Input is capped at 2 MB of text. This tool implements a single-alphabet shift cipher only — it doesn't cover polyalphabetic ciphers like Vigenère, which apply a different shift to each letter based on a repeating keyword.

Frequently asked questions

Is my text uploaded anywhere?: No. The shift is applied entirely in your browser; nothing is sent to a server.
What's the difference between ROT13 and a Caesar cipher?: ROT13 is a Caesar cipher with the shift fixed at 13 — the point at which the alphabet's 26 letters split exactly in half, making it its own inverse. This tool supports any shift from 0 to 25, with ROT13 as the default.
Are spaces, numbers and punctuation affected?: No — only letters A–Z and a–z are shifted. Everything else, including spaces, digits and punctuation, passes through unchanged, and letter case is always preserved.
Is this cipher actually secure?: No — a Caesar shift has only 26 possible keys and is trivial to brute-force. It's used for light obfuscation (spoiler text, puzzle hints) and as a teaching example, never for real security.